Reddit mentions of A Fanatic's Guide to Ear Training and Sight Singing Volume One

Common fitness-related concepts of discipline and progressive overload are ones that I think can be applied to any skill, including music. And the idea of having a set routine allows you to track progress and raise difficulty in a manageable and controlled way.

I think ear training fits this model very well. There is a book by Bruce Arnold called "Fanatic's Guide to Ear Training and Sight Singing" that has a routine you could apply to this. The basic idea is to progressively memorize the relationship between a tonality (i.e. a major or minor key) and each of the 12 pitch classes (relative to the key).

For example, you start off with the relative "do", the tonic. You are given a major chord in a random key and your job is to sing "do". With each chord you're given, you repeat by singing "do" in that chord. For example, if you're given a D major triad, you sing a D, then you might be given an G# major triad, and you sing G#. You keep doing this until singing "do" from hearing the chord requires no thought, no mental shortcut or mnemonic. Once you master that, you move on to "sol". Again, it has to be automatic. No relying on mnemonics like singing the Star Wars theme in your head. With time, it becomes second nature. As you continue on, it gets harder, especially when you start singing scale degrees that aren't the chord, like "re" or "la". Then it gets a lot harder when you start singing notes that aren't in that key, like "di" or "fi".

You can also do it the opposite way - hearing a note against a chord and identifying the scale degree.

This whole progression of ear training is quite difficult and requires a lot of time and discipline, but the benefits are huge. It helps in so many other aspects of musicianship. Being able to identify scale degrees will improve your ability to analyze and understand music as you're listening to it. For example, for much of contemporary or pop music, you'll be able to analyze the harmony just by identifying the scale degrees of the bass-line. It will make it easier to compose if you have music in your head. And of course, if you're a singer, it'll make it much easier to sight sing.

EDIT: here's a link to the book in case any one is interested: https://amzn.com/189094419X

I've been playing over twenty years and went to school for guitar, but nothing has improved my playing more than the sort of ear-training I've been doing for a little over a year now. I used A Fanatic's Guide to Ear Training. Also, I use Jazz Scales for Guitar to practice playing modes and scales around a circle of fourths chord progression so I really know my way around the fretboard.

How well can you play by ear though? Can you play back a solo phrase effortlessly and correctly, immediately, and with zero mistakes? If not I'd be focusing on ear training. Movable 'do' major/'la' minor solfege is the best way to improve your ear to fretboard link (not interval training!). It'll take work, but once you have this skill, improvisation is only limited by your imagination.

You could start with something like this (it doesn't have all the answers but it'll definitely put you on the right track): https://www.amazon.com/Fanatics-Guide-Training-Sight-Singing/dp/189094419X

Once you've started serious ear training, continue learning your favourite guitar parts by ear.

Oh and as Subman said, learn to read simple sheet music. Many beginner guitar methods will get you going in this regard.